The present invention relates to a method and an apparatus for producing a color conversion table that is used when performing color conversion of data of a colored picture.
As a known method color printing using a digital color printer there is known a method wherein simulation is performed of a typographical printing. Namely, by printing using a digital color printer digital color data of cyan (C), magenta (M), yellow (Y) and black (K) that are generally used in the typographical printing, it is possible to estimate the finish of a printing or check the contents of the printing without actually preparing a form of the printing or performing actual printing by a printing machine.
However, ordinarily, a desired reproduction color itself is not obtained with mere printing performed by the digital color printer. Namely, this is because the color of ink used for typographical printing and the color obtained by color mixing are generally different from the color printed by the digital color printer, with respect to the same color data. In view of this, when attempting to obtain a reproduction color used for typographical printing by the printing method performed by the digital color printer, there is a method of performing color conversion of the input color data so as to obtain the same reproduction color as the color that is given when typographical printing has been performed based on use of the same color data therein. As a method of performing the color conversion, there is a known method wherein color conversion is performed with reference to a color conversion table.
FIG. 2 is a constructional view illustrating a case where color conversion is performed reference referring to the color conversion table. Reference numeral 10 denotes a host computer that prepares and edits digital color data used in the typographical printing, 11 denotes color data memory means that stores therein this color data, 12 denotes color conversion means that converts the color data that has been input, 13 denotes a color conversion table used by the color conversion means, and 14 denotes a digital color printer that performs color printing by the use of post-color-conversion data. The color data prepared and edited by the host computer 10 or the color data stored in the color data memory means 11 is sent to the color conversion means 12. Usually, color data is composed of (C), (M), (Y) and (K) (cyan, magenta, yellow and black, respectively) components. The color conversion means 12 converts the (C), (M), (Y) and (K) color data to (C') , (M'), (Y') and (K') so that printed-color correspondence may hold true when being printed by the digital color printer 14. In the color conversion table 13, there are stored beforehand output values to be generated in response to the input values that have been determined considering the characteristic of the target typographical printing and the characteristic of the digital color printer used in the printing process.
In the color conversion table, although a number of values with respect to all input value cases may be prepared previously, the a reduced number of values for the purposes of decreasing the memory capacity are ordinarily stored therein and interpolation processing is performed when reference is made to the color conversion table. When printing the color (C'), (M'), (Y') and (K') converted by the color conversion means 12 by the use of the digital color printer 14, there are obtained the same colors as those which are obtained when plating or typographical printing has been performed using the initial (C), (M), (Y) and (K) color data.
Next, a method of producing the above-described color conversion table will be explained. Here, an explanation will be given of a case where each of the input and output of the color conversion is composed of, for example, three components such as (C), (M) and (Y). FIG. 3 is a block diagram illustrating a conventional method of producing the color conversion table. Reference numeral 1 denotes a target color characteristic description table defined with respect to a relationship between the input color data of a target color output apparatus and the color value of a color output therefrom and 2 denotes a reproduction color characteristic description table defined with respect to a relationship between the input color data of a reproduction color output apparatus and the color value of a color output therefrom. Here, the target color output apparatus is a color output apparatus whose color is a target with respect to which color coincidence is to be achieved and, for example, is a printing machine. Also, the reproduction color output apparatus is a color output apparatus wherein actual color printing is to be performed and, for example, is a digital color printer. As the color value there is used, for example, a CIE 1976 (L*a*b*) which is well known as a uniform color space. The target color characteristic description table 1 or reproduction color characteristic description table 2 can be produced by directly measuring by a colorimeter the color that is output from the target color output apparatus or reproduction color output apparatus. Also, the table 1 or 2 can be produced by computing the measured color values with the use of a theoretical formula such as a Neugebauer equation. Reference numeral 3 denotes an address generating means A for generating an address value with respect to the target color characteristic description table 1, and reference numeral 4 denotes an address generating means B for generating an address value with respect to the reproduction color characteristic description table 2. Reference numeral 5 denotes evaluation means for performing evaluation on the basis of the color values output by the target color characteristic description table 1 and reproduction color characteristic description table 2. Reference numeral 6 denotes determination means for performing determination on the basis of the evaluated results of the evaluation means 5, and Reference numeral 7 denotes color conversion table data memory means in which there is stored beforehand computed color conversion table data.
First, though the control of control means (not illustrated), the address generating means A 3 generates the (C), (M) and (Y) values used as the input values of the color conversion table while sequentially looping these values. The target color characteristic description table 1 sends to the evaluation means 5 the color values corresponding to the (C), (M) and (Y) values generated by the address generating means A 3. Also, the control means causes the address generating means B 4 to generate all (C), (M) and (Y) combination values corresponding to the input of the reproduction color characteristic description table 2 with respect to one set of (C), (M) and (Y) values generated by the address generating means A 3. The reproduction color characteristic description table 2 sends to the evaluation means 5 the color values corresponding to the (C), (M) and (Y) values generated by the address generating means A 3. The evaluation means 5 compares the color values obtained from the target color characteristic description table 1 and those obtained from the reproduction color characteristic description table 2 and evaluates based on this comparison. As the evaluation method, there is computed the color difference .DELTA.E*ab that is obtained by computing the square root of the sum of the differences between the respective corresponding values of, for example, two sets of (L*, a*, b*). The determination means 6 makes its determination based on the use of the evaluated value of the evaluation means 5 and selects one set of color values of the reproduction color characteristic description table 2 that are the nearest to the color values obtained from the target color characteristic description table 1. As the determination method, there are selected, for example, the color value which produce a minimum color difference .DELTA.E*ab value determined by the evaluation means 5. Next, the color data values (C'), (M') and (Y') which give the color values selected by the determination means are stored in the color conversion table data memory means 7 as post-conversion values of the (C), (M) and (Y) values of the color conversion table obtained from the address generating means A 3. The above-described operation is performed while sequentially looping the (C), (M) and (Y) values generated by the address generating means 3. As a result, in the color conversion table data memory means 7 there is produced a color conversion table necessary for color conversion.
Incidentally, although an explanation has been given of a case where the address generating means B 4 generates all combinations (C), (M) and (Y) values corresponding to the input of the reproduction color characteristic description table 2 with respect to one set of (C), (M) and (Y) values generated by the address generating means A 3, if the same results are obtained, the address generating means B 4 is not needed at all times to generate all combinations (C), (M) and (Y) values.
As explained above, in a case where the input of the reproduction color characteristic description table 2 is composed of three parameters such as (C), (M) and (Y), since there is the nature that a particular color is determined by a single combination of (C), (M) and (Y), it has been possible to determine optimum values of (C), (M) and (Y) by the determination means.
However, in a case where the input of the reproduction color characteristic description table 2 has four parameters, such as (C), (M), (Y) and (K), the combination of optimum (C), (M), (Y) and (K) values with respect to a particular color is not singularly determined and many combinations are inconveniently obtained. For this reason, there has been the problem that it has thus far been impossible to determine a single optimum value from the conventional color values.
The present invention has been made in view of the above-described respects and has as an object the production of a color conversion table whose output is (C), (M), (Y) and (K) easily and highly precisely.